Ion-selective electrodes (ISEs) are increasingly important tools in medicine and research. The most common example is the pH electrode which detects hydrogen ions; others include sensors for potassium, sodium, chloride, or other ions. Currently, valinomycin is the ligand with the most specificity for potassium. Potassium-sensitive membranes which can serve to isolate on internal reference electrode solution from a test solution are made by embedding or mixing valinomycin in the membrane along with several other membrane components such as a polymer matrix, solvents, and plasticizers. The difficulty with this current method is that during use, both ligand and plasticizers continuously leach out of the membrane leading to an unstable situation. This gives rise to changes in the electrical properties of the membrane and the electrode which in turn lead to electrode drift, baseline shift and the like.
Since 1972, researchers have been attempting to make an all solid state sensor based on the principles of the ISE. Attempts to eliminate the internal reference solution have generally focused on depositing the ion-selective membrane in direct contact with the internal reference electrode. Methods have varied from directly coating solid conductors--wires or planar metalized surfaces--to coating the insulator on the gate of a field effect transistor (FET).